Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by structure of the crystal. The zeolites are referred to as "molecular sieves" because the uniform pore size of a zeolite material may allow it to selectively adsorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated D. M. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York., p. 5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which completes the electrovalence makeup of the empirical formula.
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by the zeolite. The silicon/aluminum atomic ratio of a given zeolite is often variable. Moreover, in some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. One example is ZSM-5. ZSM-5 is a member of a class of zeolites sometimes referred as medium pore zeolites. The pore sizes of medium pore zeolites range from about 5 to about 8 Angstroms. By comparison, zeolites referred to as large pore zeolites have pore sizes which are greater than 7 Angstroms.
The zeolite ZSM-5 is an example of a zeolite which has a silica:alumina ratio which can range up to infinity. Moreover, the zeolite ZSM-5 has been described as synthesized with elements other than silicon and aluminum in the framework.
The large pore zeolites, having pore sizes greater than about 7 Angstroms, include inter alia naturally occurring faujasite and synthetic zeolites such as zeolite X, L and Y. When, for example, the zeolite Y is dealuminated to increase the silica:alumina ratio of the as-synthesized zeolite, a change in the unit cell structure occurs. The efficacy of methods for dealumination of Zeolite Y varies. However, certain zeolite Y species on dealumination yield structures which are referred as ultrastable Y, which exhibits the same X-ray diffraction pattern as zeolite Y but which has a smaller cell structure. This ultrastable Y exhibits greater stability than its higher aluminum counterpart and different selectivity from that counterpart.